1. Open the Content Type of the Content you are listing (it is important that you do this in the content type so it is locked to this content type)
  2. Create a new field that is a "select dynamic"
  3. Change the Query to "free"
  4. Enter the following code
    SELECT b.avatar AS text, a.id AS value
    FROM #__users AS a
    LEFT JOIN #__cck_store_form_users AS b ON b.id = a.id
  5. Make sure the storage is Standard > Article > created_by
  6. Save your field.
  7. Add your field to the view you want the Avatar to display on (Content, Item or List).
  8. Apply the "HTML" typography and write the HTML code - remember to replace my_field with your actual field name:

<img src= "$cck->getText('my_field')">

With thanks to Alain L for the initial question on the forums